Original paper
Pre-plan parameters predict post-implant D₉₀ ≥ 140 Gy for ¹²⁵I permanent prostate implants

Purpose: To find permanent prostate implant (PPI) pre-plan dosimetric parameters that predict post-implant D₉₀ ≥ 140 Gy.

Material and methods: Pre-plans were evaluated for 504 patients undergoing PPI with ¹²⁵I seeds for low or intermediate risk prostate cancer. Baseline patient and disease factors, numbers of seeds, ratios of number of seeds to available positions (occupancy proportion), and distances between the 100% isodose line and edge of the prostate (margin) planned for the whole prostate (WP), superior (S), inferior (I), anterior (A), and posterior (P) halves, SA, SP, IA, and IP quarters, and superior (S_T), inferior (I_T), and middle (M_T) thirds, and anterior (A_T) and posterior (P_T) middle one-sixth segments were analyzed by post-implant D₉₀ subset (≥ 140 Gy vs. < 140 Gy).

Results: 20% had post-implant D₉₀ < 140 Gy (mean: 128.0 Gy, range: 97.5-139.2) vs. ≥ 140 Gy (mean: 154.4 Gy, range: 140.0-193.5). The D₉₀ ≥ 140 Gy subset had larger AT and IA segment mean numbers of seeds (p = 0.01, 0.046), larger WP, S, A, SA, S_T, A_T, and M_T segment mean margins (p = 0.01, 0.01, 0.001, 0.0001, 0.03, 0.005, 0.02), and lower P_T segment occupancy proportion (p = 0.004). On multivariate analysis, independent predictors of post-implant D₉₀ ≥ 140 Gy were increased SA mean margin, no pre-implant 5-α-reductase inhibitor, higher pre-plan D₉₀, decreased P occupancy proportion, no pre-implant hormone therapy, and decreased SP mean margin.

Conclusions: Higher occupancy proportion and larger margins anteriorly and reduced occupancy proportion, and smaller margins posteriorly on PPI pre-plans predict post-implant D₉₀ ≥ 140 Gy.